Re: RFA: don't try to compare IEEE NaN's

[Michael Snyder <msnyder@cygnus.com>]

Jim Blandy wrote:
> 
> Eli Zaretskii <eliz@is.elta.co.il> writes:
> > My assumption was that whoever wrote the test wanted to see that GDB
> > doesn't lose bits due to all kinds of conversions that are going under
> > the hood.  If that is true, you want to make sure the value you work with
> > has the same bit pattern you wanted it to have.  If not, you don't really
> > know what you are testing here; for example, imagine an (absurdly
> > unrealistic) case that the compiler turns your literal constant into an
> > all-zero bit pattern, or into a NaN.  Then you are back to square
> > one.
> 
> What you're saying is that, between this:
> 
>         union {
>           float f;
>           char bytes[80];
>         } u;
> 
>         for (i = 0; i < 80; i++)
>           u.bytes[i] = something interesting;
> 
> and this:
> 
>         u.f = 2.7182818284590452354;
> 
> that you're more concerned that the latter will put a NaN in u.f than
> the former.  When, in fact, the exact problem I'm trying to fix is
> that someone's first shot at the former strategy produced a NaN.

"Someone" is me.  I of course knew that FFFFFFFF would be a NaN --
I just didn't know that NaN could not be compared to itself on
some platforms.  BTW, the reason for using a union as I did, 
rather than individual char, short, int etc. variables, was to
make sure that the known bit pattern was actually larger than
the type being tested -- so that we would know if, for instance, 
GDB was testing more bits than it should.